Boron nitride is a compound consisting of boron and nitrogen in an atomic ratio of 1:1. Boron nitride is known to exist in several crystal structures when formed at atmospheric pressure. The structures are an amorphous structure (including a turbostratic structure), a hexagonal crystal and a rhombohedral (or trigonal) crystal.
Among these structures, rhombohedral boron nitride is different from hexagonal boron nitride and amorphous boron nitride in the stacking mode of layers of hexagonal network formed by alternatively linked boron and nitrogen. More specifically, the hexagonal crystal structure is built up by a two-layered stacking sequence, such as ABABAB . . . ; rhombohedral crystal structure is composed of a stack of the period of three layers, such as ABCABCABC . . . ; and amorphous boron nitride does not have such regularity or periodicity in its stacked structure.
On the other hand, there have been known, as high pressure phase boron nitride, wurtzite type boron nitride and cubic boron nitride (or also referred to as zinc blend type boron nitride) and, of these two, particularly cubic boron nitride is highly valuable as an industrial material because of its super-high hardness and high thermal conductivity. It can be explained that the structures of wurtzite type boron nitride and cubic boron nitride are formed when the atoms constituting the hexagonal network structure of atmospheric pressure phase boron nitride alternatively deviate from the plane of the hexagonal network to form a zig-zag network in which wurtzite type boron nitride has a two-layered stacking sequence (ABABAB . . . . ) and cubic boron nitride has a three-layered stacking repetition (ABCABCABC . . . ). Therefore, it can be assumed from the comparison between the stacking sequences of the atmospheric phase boron nitride and the high pressure phase boron nitride that atmospheric phase rhombohedral boron nitride is liable to transform into cubic boron nitride. Factually, the validity of this assumption was experimentally justified by explosive shock compression described in Journal of American Ceramic Society, Vol. 65, No. 10, page c-162 (1982). As set forth above, rhombohedral boron nitride is very suitable as a starting material for the preparation of cubic boron nitride and thus has become increasingly important in industrial applications.
In Compt. Rend., Vol. 246, page 1866 (b 1958), it has been reported that the rhombohedral boron nitride set forth above can be prepared in powder form from the reaction of borax or boron oxide with potassium cyanide, together with hexagonal boron nitride powder.
Also, in Japanese Patent Application Laid-open No. 58-74 511 and J. Crystal Growth, Vol. 52, page 285 (1981), rhombohedral boron nitride is prepared as a white wool-like material by the reaction between an oxygen-containing boron compound and cyanide gas. However, J. Crystal Growth. Vol. 52, page 285 (1981) states that the wool-like boron nitride such prepared is an agglomerate composed of whiskers (i.e., single crystal) of rhombohedral boron nitride, whiskers of hexagonal boron nitride and filaments of boron nitride of poor crystallinity.
As described above, rhombohedral boron nitride obtained by any production method heretofore known is a mixture in a powder or wool-like form containing boron nitride having crystal structure different from rhombohedral structure and is entirely different from the bulk or thin film products of pure and high density polycrystalline boron nitride of the present invention which consists essentially of rhombohedral crystals with a preferred orientation. Further, any of the known methods of producing rhombohedral boron nitride set forth above is characterized in that an oxygen-containing boron compound is used as a boron source material and is reacted with a cyanide compound. Such known methods have the following problems. First, the methods use an extremely toxic cyanide compound and thus great care should be taken in handling it. A second problem is that rhombohedral boron nitride can be obtained only in a powder form or a wool-like form. The fact that rhombohedral boron nitride can be obtained only in a powder form or a wool-like form is extremely disadvantageous for industrial application because such boron nitride can not be easily sintered and thus a sintering acceleration agent is indispensably needed to convert the material to a bulk form. The use of the sintering acceleration agent will degrade the purity of the resulting products.
As pure and high density boron nitride with a preferred crystallite orientation, there have been known boron nitride referred to as vapor-deposited boron nitride or as pyrolytic boron nitride and many studies on those have been reported. Also, these boron nitrides have been produced on an industrial scale. For example, U.S. Pat. No. 3,152,006 describes a method for depositing turbostratic and/or hexagonal boron nitride in which reactant gases of boron halide and ammonia are mixed and reacted at a temperature of 150.degree. to 200 .degree. C. and the gaseous products formed by the reaction are then introduced into a reaction chamber to deposit boron nitride onto the surface of a substrate being maintained at a high temperature in the reaction chamber. Further, Japanese Patent Application which has been laid open to public inspection under 55-47 379 discloses that hexagonal boron nitride of high crystallinity can be formed at low temperatures by using iron as a substrate. Further studies on the production of boron nitride by vapor deposition have been also reported by Basch and Shiff in Material Design Engineering, February(1964), page 78; by Male and Salanoubat in Proceedings of the Seventh International Conference on Chemical Vapor Deposition, page 391 (1979); by Takahashi et al. in Journal of the Ceramic Society of Japan, Vol. 89, page 63 (1981); and by Hirayama and Shono in Journal of Electrochemical Society, Vol. 122, page 1671 (1975). However, these references all state that the boron nitrides produced by these known vapor deposition methods are amorphous boron nitride (including boron nitride with turbostratic structure) and/or hexagonal boron nitride, but not polycrystalline boron nitride comprising rhombohedral crystals referred to in the present invention.
Under such circumstances of the foregoing prior art, the object of the present invention is to provide highly pure polycrystalline boron nitride comprising rhombohedral crystals in a massive form or in a thin film form by vapor deposition. More specifically, an object of the present invention is to provide polycrystalline rhombohedral boron nitride of high purity and of high density which is industrially very useful as a starting material for cubic boron nitride and further the present invention is also directed to a method of producing such boron nitride.